1. Field of the Invention
The present invention relates to an optical device such as an optical filter, photodetector, semiconductor laser, or optic amplifier, using a wavelength selective photocoupler comprising two waveguides coupled by a diffraction grating.
2. Related Background Art
A conventional wavelength selective photocoupler consisted of two optical waveguides formed on the same substrate, as described in R. C. Alferness et al.: Applied Physics Letters, 33, P161 (1978), Japanese Laid-Open Patent Application No. 61-250607, or Miki et al., Electronic Communications Institute study report OQE81-129, for example.
FIG. 1 shows the construction of such a conventional wavelength selective photocoupler. In the same figure, two waveguides 191, 192 are formed with line widths or heights W.sub.1, W.sub.2, and refractive indexes n.sub.1, n.sub.2, which are different from each other, as shown. Thus, they have also different distribution relations between the wavelength of guided light propagating through the respective optical waveguides 191, 192 and the propagation constant. The propagation constants of the two waveguides coincide for guided light on a specific range of wavelength, and the optical coupling takes place between the two waveguides 191, 192. In other words, only the light within the specific range of wavelength is selected, and the optical power can be transferred between the waveguides 191, 192.
Such a photocoupler has been used for an optical filter which performs the multiplexing or demultiplexing of waves between the signal light and the light of specific wavelength by using the transfer of this optical power.
However, the optical filter as above mentioned had the spectral transmissivity characteristic containing the non-negligible side lobe on both sides of the transmission band (main lobe) including a central wavelength, as shown in FIG. 2. The existence of the side lobe brought about the cross talk of optical signals when this optical filter was used in the optical communication system using the wavelength division multiplexing method. If there is provided a sufficient separation of wavelengths to avoid the cross talk, the number of communication channels is decreased. The side lobe of the filtering characteristic on such a photocoupler was a factor in preventing the enhancement of performance on an optical device using this photocoupler.
A method for suppressing the side lobe as above mentioned, in which an interval between waveguides 193, 194 constituting the photocoupler is gradually changed as shown in FIG. 3, was proposed in R. C. Alferness et al.: IEEE Journal of Quantum Electronics, QE-14, No. 11, p. 843 (1978).
However, this method had a disadvantage that it was difficult to form the waveguide 193 curvilinearly, and further difficult to manufacture the photocoupler of a type of laminating waveguides in the direction of thickness.
A photocoupler having a sharp wavelength selectively and little loss of quantity of light was proposed in R. C. Alferness et al., "Grating Assisted in GaAsP/INP vertical co-directing coupler filter" 1989 technical digest series vol. 4, pp 215-218. This photocoupler had two guiding layers with different guided modes from each other laminated on a substrate and coupled optically by a diffraction grating. The present inventor has proposed an optical device using this photocoupler in U.S. Ser. No. 491,203 filed on Mar. 9, 1990.